local nmap = require "nmap"
local shortport = require "shortport"
local stdnse = require "stdnse"
local string = require "string"
local table = require "table"

description = [[
Extracts a list of Citrix servers from the ICA Browser service.
]]

---
-- @usage sudo ./nmap -sU --script=citrix-enum-servers -p 1604
--
-- @output
-- PORT     STATE SERVICE
-- 1604/udp open  unknown
-- | citrix-enum-servers:
-- |   CITRIXSRV01
-- |_  CITRIXSRV02
--

-- Version 0.2

-- Created 11/26/2009 - v0.1 - created by Patrik Karlsson <patrik@cqure.net>
-- Revised 11/26/2009 - v0.2 - minor packet documentation


author = "Patrik Karlsson"
license = "Same as Nmap--See https://nmap.org/book/man-legal.html"
categories = {"discovery", "safe"}


portrule = shortport.portnumber(1604, "udp")

--
-- process the response from the server
-- @param response string, complete server response
-- @return string row delimited with \n containing all published applications
--
function process_server_response(response)

  local packet_len, pos = string.unpack("<I2", response)
  local server_name
  local server_list = {}

  if packet_len < 40 then
    return
  end

  -- the list of published applications starts at offset 40
  local offset = 41

  while offset < packet_len do
    server_name, pos = string.unpack("z", response:sub(offset))
    offset = offset + pos - 1
    table.insert(server_list, server_name)
  end

  return server_list

end


action = function(host, port)

  local packet, counter, socket
  local query = {}
  local server_list = {}

  --
  -- Packets were intercepted from the Citrix Program Neighborhood client
  -- They are used to query a server for its list of published applications
  --
  -- We're really not interested in the responses to the first two packets
  -- The third response contains the list of published applications
  -- I couldn't find any documentation on this protocol so I'm providing
  -- some brief information for the bits and bytes this script uses.
  --
  -- Spec. of response to query[2] that contains a list of published apps
  --
  -- offset size   content
  -- -------------------------
  -- 0      16-bit Length
  -- 12     32-bit Server IP (not used here)
  -- 30     8-bit  Last packet(1), More packets(0)
  -- 40     -      null-separated list of applications
  --
  query[0] = string.char(
  0x1e, 0x00, -- Length: 30
  0x01, 0x30, 0x02, 0xfd, 0xa8, 0xe3, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00
  )

  query[1] = string.char(
  0x2a, 0x00, -- Length: 42
  0x01, 0x32, 0x02, 0xfd, 0xa8, 0xe3, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x01, 0x00, 0x02, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
  )

  counter = 0

  socket = nmap.new_socket()
  socket:set_timeout(5000)

  local try = nmap.new_try(function() socket:close() end)
  try(socket:connect(host, port))

  -- send the two first packets and never look back
  repeat
    try(socket:send(query[counter]))
    packet = try(socket:receive())
    counter = counter + 1
  until (counter>#query)

  -- process the first response
  server_list =  process_server_response( packet )

  --
  -- the byte at offset 31 in the response has a really magic function
  -- if it is set to zero (0) we have more response packets to process
  -- if it is set to one (1) we have arrived at the last packet of our journey
  --
  while packet:sub(31,31) ~= "\x01" do
    packet = try( socket:receive() )
    local tmp_table = process_server_response( packet )

    for _, v in ipairs(tmp_table) do
      table.insert(server_list, v)
    end
  end

  if #server_list>0 then
    nmap.set_port_state(host, port, "open")
  end

  socket:close()

  return stdnse.format_output(true, server_list)

end
